To meet the demands of user terminals on the full coverage and the high capacity of a system, many Relay Stations (RS) may be used in future mobile communication systems. Relevant definitions of the RSs are provided as follows:
A relay station is a node having the function of forwarding the data from a user terminal or a control signaling between node and node. Wherein, between node and node includes: between a BS (Base station) and an RS, between a RS and a RS, and between a RS and a User Terminal (UT).
A transparent RS: it does not transmit downlink synchronous channel message, system configuration message and resource distribution broadcast message.
A non-transparent RS: the non-transparent RS transmits its own downlink synchronous channel message, and the system configuration message and resource allocation broadcast message.
An access link is a radio link starting or terminating at a UT.
A relay link is a radio link between a BS and a RS or a radio link between a RS and a RS.
Compared with the BS, the RS has advantages of flexible arrangement and low cost, thus the cost of building a network can be reduced. The introduction of the RS can expand the system capacity, enlarge the coverage area of a cell, and enhance the transmission data rate.
FIG. 1 is a schematic view of different application scenes and effects of RSs. In the actual use, the coverage area of a transparent RS completely falls within the coverage area of a BS, while the coverage area of a non-transparent RS may partly or completely beyond the coverage area of a BS. As shown in FIG. 1, the RS2 for providing hotspot coverage is a transparent RS, while the relay station RS1 for enlarging the coverage area of the BS and the RS3 for solving the shadow effect are non-transparent RSs.
To avoid mutual interference of radio signals between RSs, during actual network deployment, usually the operating frequency bands of adjacent RSs are differentiated, as the operating frequency bands of the nodes shown in FIG. 2. As shown in FIG. 2, RS1, RS2 and RS3 are the RSs dominated by the BS. The BS can use all the frequency bands in the cell, and respective RSs can use part or all of the frequency bands in the cell. The size of the frequency band used by the RS depends on the traffic volume of the user terminal served by the RS and the designing ability of the RS. RS1, RS2 and RS3 have a same frequency band 210 with the BS, the corresponding frequency bands of the BS and the respective RSs are shown with the lattice filling portion, and the same frequency band is called the common frequency band, the main functions of which include providing mutual communication between RSs; and receiving common information transmitted from the UT, such as preamble etc.
The handover procedure of serving nodes performed by UT is explained as follows: the handover refers to a procedure in which the user terminal must handover from the current communication link to an idle communication link managed by other access nodes when a user terminal moves from the coverage area of one BS or RS to that of another BS or RS during communication, or the quality of communication is degraded due to external interferences, wherein the access node is a BS or a RS.
The handover procedure of mobile communication systems can be divided into three main procedures: the measurement procedure, the judgment procedure and the execution procedure respectively, wherein,
The measurement procedure: its main function is to measure parameters needed by handover in a radio communication system and verify the result of a measurement report. The measurement procedure is mainly divided into an intra-system measurement and an inter-system measurement, and an intra-frequency measurement and an inter-frequency measurement. The measurement procedure mainly relates to parameters such as the period and the format of the measurement report. In the current wireless communication system, the measurement mainly refers to that of the UT who detects the downlink pilot signal transmitted from the BS, and judges the quality of the downlink pilot signal. In a future mobile communication system, some serving nodes on network side, such as the transparent RS, do not have a downlink pilot signal for measurement, and in this situation, the quality of the signal between the UT and the network cannot be measured.
The judgment procedure: its main function is to give a judgment result according to the demands of parameters about network and service and by referring to the corresponding thresholds and the measurement result, and finally determine whether the UT handovers and the target cell of handover. The design of the judgment procedure specifically embodies various handover rules in a handover algorithm and also specifically reflects the system performance.
The execution procedure: its main function is to enable the UT to establish a connection with the target cell by interacting signaling between the target cell and the UT when the judgment procedure judges that corresponding handover of the UT is needed, and to assign corresponding resources to the user, thereby completing the entire handover procedure.
According to the above description, in the prior art, when some serving nodes on network side, such as the transparent RS, do not have the downlink pilot signal for measurement, the user terminal cannot measure the quality of the signal between the UT and the network, thus is unable to perform cell handover.